Democrats don’t resign.
Period.
They do if the Party determines they're a liability.
That is based on if Republicans are getting votes by tying the boat-anchor to them in election advertisements.
If that happens, they're gone and their name and everything else about them goes into the memory hole.
Usually, they have to agree to that. One thing that helps them decide to agree is if they are given a cushy sinecure, like a BS job in some foundation, or put in some no-show professor position in an A-A studies department, or some such.